1. Technical Field
This invention generally relates to electronic signal switching devices, and more specifically to electronic radio frequency signal switching devices.
2. Background
Electronic signal switches are used in a wide variety of applications. One type of signal switch in common use is a field effect transistor (FET) that is actively controlled through a gate terminal to block or pass an electrical signal connected in series with source and drain terminals of the FET (in another mode of operation, a FET also may be used to modulate an electrical signal in response to a varying signal on the gate terminal).
Field effect transistors may be fabricated in various technologies (e.g., standard bulk silicon, silicon-on-insulator, silicon-on-sapphire, GaN HEMT, GaAs pHEMT, and MESFET processes) and are commonly represented in schematic diagrams as an idealized device. However, in many applications, particularly in radio frequency (RF) circuits, the structure and materials of a FET switch may have significant effects on its own operation (e.g., with respect to bandwidth, isolation, and power handling) and the presence of a FET switch may have significant effects on other components in a circuit. Such effects arise in part because a “CLOSED”/“ON” (low impedance) FET has a non-zero resistance, and an “OPEN”/“OFF” (high impedance) FET behaves as a capacitor due to parasitic capacitances arising from the proximity of various semiconductor structures, particularly within the close confines of an integrated circuit (IC). Large signal behaviors affecting power handling may also arise from other characteristics of a FET, such as avalanche breakdown, current leakage, accumulated charges, etc. Accordingly, the actual in-circuit behavior of a FET must be taken into account when designing FET based circuitry.
One use of FET switches is within RF frequency signal switching devices. For example, FIG. 1A is a schematic diagram of a prior art 3-port reflective signal switching device 100 for selectively coupling one of two terminal ports 102A, 102B (shown series connected to respective external loads RF1, RF2) to a common port 104 (shown series connected to an external load RFC). Accordingly, the signal switching device 100 may be regarded as a single-pole, double-throw (SPDT) switch. In other configurations, more than two terminal ports (a 1×N switch) and more than one common port may be included (an M×N switch). Between the common port 104 and each terminal port 102A, 102B are respective FET series switches 106A, 106B; the FET series switches 106A, 106B may vary in size, for example, to accommodate different power levels. Between each terminal port 102A, 102B and its respective series switch 106A, 106B are respective FET shunt switches 108A, 108B, coupled to circuit ground. Such a switching device 100 may be used, for example, to selectively couple RF signals between two antennas respectively connected to the terminal ports 102A, 102B and transmit and/or receive circuitry connected to the common port 104. For RF signals, each load/source impedance RF1, RF2, RFC would typically have a nominal impedance of 50 ohms by convention.
In operation, when terminal port 102A is to be coupled to the common port 104, series switch 106A is set to a low impedance ON state by means of control circuitry (not shown) coupled to the gate of the FET series switch 106A. Concurrently, shunt switch 108A is set to a high impedance OFF state. In this state, signals can pass between terminal port 102A and the common port 104.
For the other terminal port 102B, the series switch 106B is set to a high impedance OFF state to decouple the terminal port 102B from the common port 104, and the corresponding shunt switch 108B is set to a low impedance ON state. One purpose of setting the shunt switch 108B to ON—thus coupling the associated terminal port 102B to circuit ground—is to improve the isolation of the associated terminal port 102B (and coupled circuit elements, such as antennas) through the corresponding series switch 106B. For switching devices with more than two terminal ports, the series switch and shunt switch settings for the “unused” (decoupled) terminal port to common port signal paths typically would be set to similar states.
FIG. 1B is a diagram showing an equivalent circuit model of the prior art 3-port signal switching device of FIG. 1A. Shown is a circuit configuration 120 in which terminal port 102A has been coupled to the common port 104; accordingly, series switch 106A and shunt switch 108B are set to a low impedance ON state, while series switch 106B and shunt switch 108A are set to a high impedance OFF state. In this configuration, series switch 106A is modeled as a resistor 126A having a resistance value of Ron (i.e., the CLOSED or ON state resistance of a FET), shunt switch 108A is modeled as a capacitor 128A having a capacitance of Cshunt (i.e., the OPEN or OFF state capacitance of a FET), series switch 106B is modeled as a capacitor 126B having a capacitance of Coff, and shunt switch 108B is modeled as a resistor 128B having a resistance value of Rshunt. As in FIG. 1A, with the illustrated circuit configuration, signals can pass between terminal port 102A and the common port 104.
FIG. 1C is a diagram showing a simplified equivalent circuit model 130 corresponding to the circuit configuration 120 shown in FIG. 1B. Series switch 106B (modeled as a capacitor 126B in FIG. 1B) is OFF. The corresponding shunt switch 108B (modeled as a resistor 128B in FIG. 1B) is ON, thus having a very low impedance and coupling terminal port 102B to circuit ground. Since Rshunt has a very low impedance, the resistor equivalent 128B in FIG. 1B may be more simply modeled as a conductor (short) to circuit ground and is thus shown in dotted-line resistor form. Therefore, the two equivalent circuit elements 126B, 128B of FIG. 1B may be modeled as a single capacitor 126B′ having a capacitance of Coff. Similarly, since series switch 106A (modeled as a resistor 126A in FIG. 1B) is ON and Ron is a very low impedance, series switch 106A may be more simply modeled as a conductor. Accordingly, the resistor equivalent 126A in FIG. 1B is shown in dotted-line resistor form, leaving OFF shunt switch 108A (modeled as a capacitor 128A with a capacitance of C shunt) connected in parallel with the external load RF1. As in FIG. 1A and FIG. 1B, with the illustrated circuit configuration, signals can pass between terminal port 102A and the common port 104, as shown by dotted line signal path 132.
The simplified equivalent circuit model 130 can be used to evaluate the insertion loss (IL) bandwidth of the circuit model 130. In this example, the 3 dB IL bandwidth is proportional to 1/(Rport*(Coff+Cshunt)) [where Rport is the load resistance at the RF1 and RFC ports], which is typically limited to below 13 GHz in current silicon IC technology.
The bandwidth of conventional radio frequency switching devices of the type shown in FIGS. 1A, 1B, and 1C is limited by the parasitic capacitance from the Cshunt equivalent components. This invention in various embodiments addresses this limitation to improve the bandwidth of RF switching devices as well as the signal isolation and power handling of such switching devices.